1. Field of the Invention
The present invention relates to an apparatus for detecting variation abnormality in air-fuel ratio among cylinders of a multicylinder internal combustion engine, and in particular, to an apparatus that detects abnormality (imbalance abnormality) in which the air-fuel ratio of one cylinder deviates relatively significantly from the air-fuel ratio of the remaining cylinders.
2. Description of the Related Art
In general, an internal combustion engine with an exhaust purification system utilizing a catalyst efficiently removes harmful exhaust components using the catalyst and thus needs to control the mixing ratio between air and fuel in an air-fuel mixture combusted in the internal combustion engine, that is, the air-fuel ratio. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the internal combustion engine to perform feedback control to make the detected air-fuel ratio equal to a predetermined air-fuel ratio.
On the other hand, a multicylinder internal combustion engine normally controls the air-fuel ratio using identical controlled variables for all cylinders. Thus, even when the air-fuel ratio control is performed, the actual air-fuel ratio may vary among the cylinders. In this case, if the variation is at a low level, the variation can be absorbed by the air-fuel ratio feedback control, and the catalyst also serves to remove harmful exhaust components. Consequently, such a low-level variation is prevented from affecting exhaust emissions and from posing an obvious problem.
However, if, for example, fuel injection systems for any cylinders become defective to significantly vary the air-fuel ratio among the cylinders, the exhaust emissions disadvantageously deteriorate. Such a significant variation in air-fuel ratio as deteriorates the exhaust emissions is desirably detected as abnormality. In particular, for automotive internal combustion engines, there has been a demand to detect variation abnormality in air-fuel ratio among the cylinders in a vehicle mounted state (on board) in order to prevent a vehicle with deteriorated exhaust emissions from travelling.
A possible method for detecting variation abnormality in air-fuel ratio among the cylinders involves calculating a parameter correlated with the degree of variation in output from the air-fuel ratio sensor and comparing the calculated parameter with a predetermined determination value to detect abnormality.
On the other hand, the output characteristics (gain, responsiveness, and the like) of an air-fuel ratio sensor actually installed in an internal combustion engine vary between tolerance upper-limit products and tolerance lower-limit products due to manufacturing variations and the like. Hence, the calculated value of the parameter corresponding to the same degree of variation in air-fuel ratio varies depending on the air-fuel ratio sensor.
On the other hand, a desired value for the degree of variation in air-fuel ratio which needs to be determined to be abnormal may be legally specified. In such a case, the determination value is specified in view of the desired value.
However, it has been found that not all air-fuel ratio sensors can meet the desired value because of the variation among the air-fuel ratio sensors. That is, it has been found that the tolerance upper-limit products allow abnormality to be detected when the parameter is smaller than the desired value, whereas the tolerance lower-limit products may fail to allow abnormality to be detected unless the parameter exceeds the desired value.
Thus, the present invention has been developed in view of the above-described circumstances. An object of the present invention is to provide an inter-cylinder air-fuel ratio variation abnormality detection apparatus that allows even an air-fuel ratio sensor corresponding to a tolerance lower-limit product to suitably and adequately detect variation abnormality.